What does Mark 10:6 mean?

Jesus makes this comment in the context of God's plan for marriage, but modern concerns demand a broader analysis of gender. God made humans to be male or female in sex and gender. Sin corrupts. It corrupts nature, genetics, social mores, and everything humans touch.

Transgenderism and the idea of a continuum of genders is a sinful misinterpretation of God's creation. Science is unsure as to why some people feel they are a different gender than their biology. Science has found that some of these situations are caused by specific mental illnesses. But even if the feeling has a chemical or hormonal cause, the Bible gives no more leeway to act on those feelings than it does a person who is naturally violent, or lazy, or prone to addiction.

In contrast, intersexualism is a genetic defect whereby a child is born with ambiguous gender traits in chromosomes, hormones, or genitalia. Intersexualism is not, in and of itself, a sin. It's a physical condition caused by the cumulative general effect of human sin on the human body. There is no condemnation in being truly, biologically, intersex.

Neither is Jesus speaking against same-sex marriage, here, at least not directly. It's worth noting that Jesus explicitly states that the plan for marriage is male and female—rather than saying something like "a loving pair" or other phrases compatible with same-sex matrimony. All the same, His comments here are not intended as a commentary on homosexuality.

Despite what some might claim, this is not a valid argument from absence; Jesus' lack of a direct statement does not mean He endorses same-sex behaviors. There are reasons Jesus didn't specifically address homosexuality: it wasn't an issue for the Israelites of His time. The audiences of Jesus' recorded teachings are primarily Jewish. People in that culture generally worked hard to obey the Mosaic law. That sin was more common in Roman culture, which is why Paul talks about it in very direct terms (Romans 1:26–27).

Finally, Jesus is not talking about how people should act. In general men and women have different personalities, strengths, and weaknesses. But if a woman isn't naturally nurturing, that doesn't mean she's a man. And if a man isn't aggressive or strong, that doesn't mean he's a woman. Much of the modern confusion over gender identities, ironically, is because of an insistence that some traits are inescapably bound to certain genders. There is room to respect the gender we are born into while we also respect our individual personalities.

Jesus is talking about something much deeper and specific: God made men and women to pair up and have good, life-long relationships. This is a universal truth for all nations, cultures, and religions. God endorses the marriages of unbelievers as much as believers. At the same time, He acknowledges that some people are better suited to remain single, whether because they wish to concentrate their efforts on God's work, or because they don't feel equipped to marry (Matthew 19:11–12).